Photosensitive resin compositions generally cure through radical polymerization, upon exposing to light. The curing reaction is inhibited by oxygen which is dissolved in the resin compositions, because the oxygen functions as a radical scavenger. It is therefore necessary that the dissolved oxygen is removed from the resin composition before exposure.
The removal of the dissolved oxygen can be conducted by placing the photosensitive resin plates in an atmosphere of carbon dioxide gas or nitrogen gas overnight before exposure in order to replace the dissolved oxygen with carbon dioxide or nitrogen gas. This method takes a long time and needs a large space for a machine. Practically conducted methods are those wherein the resin plates are preliminarily exposed (or bumped) with weak light to quench the dissolved oxygen and then subjected to a main exposure. The preliminary exposure (or bump) requires specific conditions limited for only quenching the dissolved oxygen, for example exposing time, irradiated light density and the like. In addition, if the photosensitive resin layer has a thickness of more than 0.1 mm, the weak light does not sufficiently reach to the side of the substrate, at which the removal of the dissolved oxygen is insufficient. In the following main exposure, this side will not cure sufficiently due to the remaining oxygen. Accordingly, after developing, isolated dots have barrel-like shapes and thin toward the substrate, thus resulting in the decline of durability. If an attempt is made to quench the dissolved oxygen sufficiently at this side in the preliminary exposing process, the surface which faces to light would start to unnecessarily cure. Also, if the main exposure is conducted for a longer period of time in order to sufficiently cure the side of the substrate, the etching depth of white portions and dot portions is shallow and the printing portions are thicker than negative images. It also takes a long period of time to produce printing plates and ununiformity of polymerization reaction reduces the durability of the printing plates.